This invention relates to ovens for preheating of tubular parisons or the like in preparation for a blow molding operation, and in particular it relates to means for improving the uniform application of the heat to the parisons.
In recent years, there has developed techniques whereby blow molded articles can be produced having biaxial orientation, which articles have exceptional strength and highly desirable physical properties such as clarity. This technique involves forming a tubular parison, cooling it to well below its melting point and thereafter heating it to its orientation temperature. The orientation temperature is just below the crystalline melt point in the case of crystalline materials and 40.degree. to 225.degree. F below the homogenous melt point for amorphous materials. Since the parison is cooled between its formation and its use, this technique provides the significant advantage that these two functions, i.e., forming the parison and forming the finished article from the parison may be separated in time and/or place so that the formed parison can for example be sold to others who may have facilities for forming finished articles from the parisons but who may not have the facilities for function forming the parisons. Since the functin of forming the finished articles then commences with a cold parison, this technique has become known as the cold parison technique. An apparatus for heating cold parisons to an appropriate forming temperature, and transferring the heated parisons to a blow mold and then stretching the parison and blow molding it into the form of a finished article is shown and described in considerable detail in commonly owned U.S. Pat. No. 3,765,813.
The oven is of critical importance in this overall apparatus. It represents a major portion of the overall cost of the apparatus so that ideally the parisons should be brought to the forming temperature, i.e., the orientation temperature therein as rapidly as possible. On the other hand, however, the qualitative demands of the oven are considerable in that the parisons must not only be heated to a fairly precise temperature but also they must be heated uniformly along their lengths, about their circumferences and through their wall thicknesses; and if these demands are not met, resulting in an improperly heated parison, the result will be a rejected defective article which of course represents an economic waste of machine time, not to mention the costs involved in regrinding the plastic material for further use.
The problem of heating the parisons uniformly within the oven has been attacked heretofore. Commonly owned U.S. Pat. No. 3,801,263 shows and describes a new and improved parison heating oven wherein the heated air enters at one side of the oven and flows thereacross to an exhaust plenum on the opposite side thereof. This patent also describes the concept of providing means for specifically varying the heat as applied to different axial portions of the parisons for the purpose of "programming" an intentional non-uniform temperature along the length of the parison so that in the subsequent stretching and blow molding steps certain portions of the finished article will intentionally be of a greater thickness than other portions so as to provide for example an article having a greater wall thickness at the bottom. However, even in this case the problem still exists of assuring that the heat is applied uniformly about the circumference of the parison.
One solution to the problem of heating a parison uniformly about its circumference has been to rotate the parison about its axis as it moves through the oven. This known technique normally involves mounting the parison on a rotatable member which includes a toothed wheel and providing some type of fixed means alongside the conveyor path which cooperates with this toothed wheel. One such arrangement is shown in commonly owned U.S. Pat. No. 3,740,868 wherein the toothed "starwheel" engages pins placed alongside the conveyor path. Another arrangement is shown in the Marzillier U.S. Pat. No. 3,149,373 wherein a chain is provided alongside the conveyor path and the toothed wheel is actually a sprocket which engages the openings in the chain. However, these previous arrangements for rotating the parisons about their axes have had considerable disadvantages. In either case the tooth engaging means have been stiff essentially unyielding members thereby requiring a high degree of precision both in the construction and the mounting within the oven of the tooth engaging means. Moreover, even with the highest degree or precision there would be times such as during initial engagement of the toothed wheels with their respective tooth engaging means wherein the teeth would not necessarily be perfectly aligned with the engaging means so that initially a tooth might abruptly bump rather than move neatly between the fixed vertical pieces of the engaging means. At the least this might result in a continued improper engagement between the toothed wheel and the engaging means resulting in the parison not being heated uniformly about its circumference and hence ultimately becoming a reject. At worst, however, this abrupt abutment could result in breakage to the relatively expensive toothed wheel rotating member. While this problem exists with respect to both types of tooth engaging means, it is perhaps more severe in the case of the chain type engaging means since in that case the openings for the teeth are defined not only by vertically extending elements but also by upper and lower horizontally extending elements so that precision alignment is required both horizontally and vertically.
Thus, there exists a need for a new and improved tooth engaging means for cooperating with a toothed wheel which overcomes the problems present in the prior art.